Progressing cavity pumps are in increasingly common use in the oil field for production of formation fluids to the surface. The pumps comprise a fixed outer body usually referred to as a stator which connects to the production tubing in the well. Within the stator is a rotating inner component called a rotor which in cooperation with the stator pumps the formation fluids.
The rotor is rotated by a string of drive rods that transmit torque from a prime mover at the well head. The prime mover is normally an electric motor that produces up to 100 horsepower and also generates very substantial torque. The drive rods extend from a drive head at the top of the well head down through the production tubing to the rotor.
The inside of the stator is rubber and friction is generated as the rotor spins. If the stator is not properly anchored, it will rotate in the clockwise direction (to the “right” when viewed from above) and if not checked, the tubing joints will eventually loosen and part, allowing the tool to fall to the bottom of the well. Production must then be halted until the pump is fished out. To prevent this, pump anchors are used which, when engaged against the well casing, restrict right-handed rotation of the pump.
The problem however is that the drive rods themselves store a considerable amount of energy in the form of twist. In fact, after the motor is turned on the rods might twist as many as 50 times before the stator begins to turn.
When the motor is stopped, the rods untwist to release their stored torque, and the release can be violent, made worse by the weight of the oil in the tubing from the pump to the surface, resulting in speeds approaching 20,000 rpm. Because the pump anchor has become unset in response to the counterclockwise (to the “left”) unwinding of the rods, the pump is unrestrained and whips around inside the well casing causing major damage to the pump and everything in its vicinity. The torque can also wildly spin the sheaves and pulleys that deliver torque from the motor to the drive rods which can cause additional failures and endanger anyone close by.
There are some anchors that are intended to restrain both left and right handed torque but these are typically “one set” or limited set devices and are usually referred to as “tension set anchors”. They must be recovered to the surface then refaced or redressed after each use, which limits their utility.